1. Field of the Invention
This invention relates to flowmeters for measuring the flow rate of a liquid or gaseous media. More particularly, the invention relates to flowmeters having a fluid path of variable cross sectional area.
2. Background of the Invention
This invention relates to flowmeters of the type known as variable area flowmeters. This type of flowmeter provides a weighted member movably disposed across an orifice such that the position of the weighted member determines the orifice area. The weighted member provides an essentially constant fluid head against the fluid entering the system so that the displacement of the weighted member is essentially a linear relation with the rate of volumetric flow of the fluid.
The principal advantage of a variable area flowmeter is that, at low cost, it provides a wide range of capacity with low system resistances and is essentially linear. One well known and popular form of variable area flowmeter, often called a rotameter, utilizes a weight moving vertically within a tapered tube, usually transparent, whose area increases upwardly. Among the disadvantages of this system is the inability to see the float and to read the flow when hidden by dark fluids in larger diameters toward the top of the tube. Rotameters are also subject to instability in the transition zone between laminar and turbulent flow and in many cases, viscosity influences the float response within the meter. These factors often limit accurate low end reading of rotameters to 8-12 percent of the total capacity. Certain desirable methods of applying a transducer for readout and control are limited by the geometry of a rotameter and for large flow rates, rotameters are large and costly. Finally, rotameters often experience severe float pulsation, particularly with low density gas media.
Another proven variable area flowmeter is described in my prior U.S. Pat. No. 3,691,834. This invention overcomes many of the disadvantages of the rotameter. The invention disclosed in U.S. Pat. No. 3,691,834 utilizes a snorkel for liquid media. The snorkel device adds cost and requires a built-in minimum meter pressure drop to function. The snorkel, under some circumstances, may introduce an exponential flow factor which particularly limits wide range low capacity meter designs. In addition, the "thru slot" orifice configuration is difficult to construct in widths narrow enough to provide low capacity meters. In actual practice with metal core tubes and floats, full range capacities below about 1.2 gallons per minute for liquids and 7.0 standard cubic feet per minute for gases have been demonstrated impractical within competitive costs. Extreme float pulsation with low density gas media have been observed with the design thus limiting the commercial application of this design.
Particularly with regard to measuring the rate of flow of a gaseous media, problems have been experienced in the low end of the scale range. Many flowmeters have been proposed in an attempt to combine the required characteristics of low pressure loss in the flowmeter system, together with a variable means for damping the pulsation effects within the gaseous media. Additionally, and most importantly, a long felt need has existed for a flowmeter providing an indication of the rate of flow at the low end of the scale range.
The present invention provides a flowmeter for measuring the rate of flow of a gaseous media having an improved rangeability or turn down, and particularly but not exclusively an improved sensitivity with the low end of the indicator scale range. This improved rangeability is accomplished by the provision of a connecting means extracting between a first and a second piston, the second piston being immersed in a static fluid. By varying the density of the connecting means and the density of the static fluid, the rangeability of the flowmeter is improved. Furthermore, an improved rangeability is obtained by virtue of the shape of the connecting means. The shape of the connecting means may be selected to provide a substantially linear scale.
In order to understand the principles involved in the improved flowmeter of the present invention, it will be appreciated by those skilled in the art that the float displacement which indicates a particular rate of flow, occurs when the weight of the float is balanced at that displacement by the pressure of the flowing gaseous media. This pressure is required to establish the particular flow rate across the exposed cross sectional area of the groove. This pressure acting across the area of the first piston is a force in equilibrium with the total weight of the first piston, the second piston and the connecting means. As is common to variable area flowmeters and well known in the art (NOTE: All equations assume compatible units, eliminating conversion constants.), ##EQU1## where Dg=displacement for a given cross sectional area of the groove
As=cross sectional area of the gaseous path defined by the groove PA1 Wf=weight of the float PA1 df=density of the float PA1 dm=density of the gaseous media. PA1 Vi=volume of the float immersed in the static fluid. PA1 dd=density of the static or damping fluid.
Also, the effective weight Wf of the float assembly disposed in the gaseous or liquid media is, ##EQU2## where V=the volume of the float
When the float is totally immersed in a gaseous media, the flow rate of which is to be determined, the effective weight Wf of the float is constant and the flow rate to displacement relationship is essentially the same as with conventional variable area flowmeters. Furthermore, with the majority of gaseous media, the density of the gas is so low that the weight of the float Wf is taken as, ##EQU3##
Referring to the improved flowmeter of the present invention, it will be appreciated by those skilled in the art that the connecting means connecting the first and the second pistons is variably immersed in a liquid of significant density. Therefore, relative to equations 1 and 2 explained heretofore, ##EQU4## where Vg=volume of float in gaseous media that is the nonbuoyant portion of the first and second pistons and the connecting means.
In the above situation, for a given gaseous flow rate, Vg+Vi is constant. However, in operation of the flowmeter as the flow of gas increases, the relation of Vg to Vi will be variable. This variability extends between the range from Vi&gt;Vg through Vi=Vg to a maximum value of Vi&lt;Vg.
From the above it will be appreciated by those skilled in the art that the flowmeter will not only be of variable area but also variable effective weight of the float.
In the improved flowmeter of the present invention, only the connecting means moves variably through the static or damping fluid and the essentially nonbuoyant gaseous media. This being the case according to the present invention, by varying the density of the connecting means along the length thereof, it is possible to obtain a flowmeter having a linear scale, or scales of other preferred geometry. Additionally, by providing a variable density connecting means in combination with a static fluid of cooperating density, an improved sensitivity or rangeability may be obtained with the low end of the scale range.
As will be appreciated by those skilled in the art and from the equations 1 through 4 as described hereinbefore, the effective weight of the indicator means is dependent upon the ratio of the density of the immersed portion of the connecting means and the density of the static fluid. By selecting an appropriate ratio, a flowmeter of improved range may be obtained. In addition, an improved sensitivity, particularly at the low end of the range is provided. The range as well as being dependent upon the ratio as described hereinbefore is also dependent upon the combined weight of the first and second pistons and the connecting means such that the selected combined weight will vary proportionately to the increased flow rate to be measured.
The factors upon which the range depends are thus the density of the connecting means, the density of the static fluid and the effective combined weight, respectively. The effective combined weight may be varied in several different ways including providing connecting means of various uniform diameters. Alternatively, the connecting means may be of conical configuration or of any other suitable configuration.
Therefore, it is a primary objective of the present invention to provide an improved flowmeter for measuring the flow of a gaseous media that overcomes the aforementioned inadequacies of the prior art flowmeters and provides an improvement which significantly contributes to the rangeability of the flowmeter and the linearity of the scale thereof.
Another object of the present invention is the provision of a flowmeter having an improved sensitivity at the low end of the range.
Another object of the present invention is the provision of a flowmeter having a scale of improved linearity.
Another object of the present invention is the provision of a flowmeter for measuring the rate of flow of a gaseous media, the flowmeter including a first piston and a second piston connected to the first piston by connecting means. The second piston is immersed in a static or damping fluid and the connecting means has a variable density along the length thereof.
Another object of the present invention is the provision of a flowmeter for measuring gaseous flow rates in which the density of the static fluid in relation to the variable density of the connecting means provides an improved rangeability of the flowmeter.
Another object of the present invention is the provision of a flowmeter for measuring gaseous flow rates in which the cross sectional area of the connecting means varies along the length of the connecting means.
Another objective of this invention to provide a variable area flowmeter utilizing a float within and extending from a vertically oriented cylinder within an orificed core tube which overcomes deficiencies in the prior art.
Another object of this invention is to provide a variable area flowmeter utilizing tapered slot means and slots of various geometries producible without sophisticated machinery to allow full range capabilities in terms of flow rate.
Another object of this invention is to provide a variable area flowmeter wherein an effective pulsation damping means is provided therein without the cost and complications of a pitot or snorkel as found in my prior patent, U.S. Pat. No. 3,691,834.
Another object of this invention is to provide a variable area flowmeter which also eliminates unwanted flow components and the built-in pressure drop requirements inherent to the pitot or snorkel tube.
Another object of this invention is to provide a variable area flowmeter which generates signals for remote readout flow control and the like without sacrifice of inherent visual readout at the meter.
Another object of this invention is to provide a variable area flowmeter which is linear for an extremely wide variation in flow rates and density of fluid materials.
Another object of this invention is to provide a variable area flowmeter with means to provide departure from linearity when desired over full or upper range in either increasing or decreasing increments of displacement per unit of flow rate increase.
Another object of this invention is to provide a variable area flowmeter with a demonstrated accurate flow rate reading to one percent (1%) of full capacity.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner of modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.